The present invention relates to a small 4-cycle engine used for driving a portable machine or the like, and more particularly to an intake port thereof.
The present application claims priority from Japanese Patent Application No. 2006-012875, the disclosure of which is incorporated herein by reference.
Small 4-cycle engines used for driving portable machines such as handheld blowers usually have their intake port integrally formed with the cylinder head because of the priority given to reduction of size and weight of the machine including the engine. Also, the cylinder head needs to be designed so that it is easily molded without using complex shape molds for lower cost.
More specifically, the intake port is conventionally designed to be formed so that its centerline on the entrance side is vertical to the centerline of the cylinder, while the centerline on the exit side is parallel to the cylinder centerline, i.e., the centerlines of the entrance side and the exit side of the intake port are substantially at right angles with each other. This shape allows easy fabrication of cylinder heads by die casting, contributing to reduction in cost of 4-cycle engines (see, for example, Japanese Unexamined Patent Publication No. Hei 9-170417, FIG. 5).
In general, the intake port shape should ideally be straight from the entrance to the exit so as to reduce intake resistance of fuel-air mixture. Japanese Unexamined Patent Publication No. Sho 52-66105, for example, shows a 4-cycle engine design in which the intake port shape is gently curved from the entrance to the exit.
Another known issue associated with the intake port is that condensed fuel tends to remain in the joint between the intake port and a heat insulator inserted between the carburetor for mixing liquid fuel with air and the intake port, causing possible incomplete combustion or generation of unburnt gas. One of countermeasures for this problem is disclosed, for example, in Japanese Unexamined Patent Publication No. 2000-136762, in which a conduit is provided in a protruded part of the heat insulator that is fitted with the intake port, the conduit communicating with the lower inner surface of the intake port, so as to prevent any condensed fuel from remaining inside the intake port.
The problem with the 4-cycle engine shown in Japanese Unexamined Patent Publication No. Hei 9-170417 is that the engine power output is not satisfactory due to large intake resistance of fuel-air mixture because the priority is given to reduction in size, weight, and cost of the engine and the intake port shape design still leaves scope for improvement. When used as the power source of a portable machine, the insufficient power of the engine has caused various problems in the operation of the machine.
The intake port shape of the 4-cycle engine shown in Japanese Unexamined Patent Publication No. Sho 52-66105 is hardly applicable to small engines mounted on portable machines because molding such intake port integrated with the cylinder head that entails the use of the complex and expensive metal molds, such as diagonally oriented slide molds, cores and the molding using sand molds, or the like, results in an increase in production cost and a decrease in productivity.
The problem with the 4-cycle engine shown in Japanese Unexamined Patent Publication No. 2000-136762 is that the effect of preventing condensed fuel from remaining inside the intake port is not sufficiently achieved when design changes are made to the shape of the intake port.